I. Field of the Invention
The present invention relates generally to the field of anti-cancer therapy. More particularly, it provides organic arsenic compounds and methods for their use in treating cancers such as leukemia.
II. Description of Related Art
Despite progress in leukemia therapy, most adult patients with leukemia still die from disease progression and an estimated 31,500 new cases and 21,500 deaths are expected in the year 2001. Arsenic trioxide, an inorganic compound, has recently been approved for the treatment of patients with relapsed or refractory acute promyelocytic leukemia (APL) and is being evaluated as therapy for other leukemia types. However, its use is limited by its toxicity.
Arsenic was used as a medicinal agent more than 2400 years ago in Greece and Rome, and arsenic still comprises the active ingredient in certain folk remedies, particularly in Central and Southern Asia (Bainbridge et al., 1914). The history and folklore of arsenic prompted intensive studies by many early pharmacologists. The foundations of many modern concepts of chemotherapy derive from Ehrlich's early work with arsenicals (e.g. the “silver bullet” for syphilis), and such drugs were once a mainstay of cancer chemotherapy. For example, in the early 1930's, Fowler's solution (inorganic arsenic dissolved in water) was used for controlling elevated leukocyte counts in chronic myelogenous leukemia (Forkner et al., 1931). In fact, clinical improvement of the leukemia, including control of fever, reduction of white cell count and splenomegaly, and improvement in anemia could be observed. Various arsenicals were briefly examined in early preclinical cancer screening studies at the U.S. National Cancer Institute (Tarnowski et al., 1966). However, the clinical use of arsenicals in the treatment of cancer in the U.S. virtually ceased in the early 1970's with the advent of now traditional cytotoxic drugs and radiotherapy along with reports of arsenic poisoning from chronic low-dose ingestion (Knock et al., 1971; Cuzick et al., 1987). Still, there is large body of knowledge regarding effects of arsenicals on human subjects from both medicinal and toxicological standpoints. In current therapeutics, arsenicals are important only for the treatment of certain tropical diseases, such as the use of melarsoprol, an organic compound, against African trypanosomiasis (Investigational Drug Brochure, 1987). However, Chinese reports of the striking activity of arsenic trioxide (As2O3) (Zhang et al., 1996), an inorganic compound, against APL have engendered considerable interest. This resulted in the recent approval of arsenic trioxide for treatment of patients with relapsed or refractory APL. Preliminary data from China and the recent experience in the U.S., however, suggest a role for arsenic trioxide in the other hematologic cancers as well. Consequently, the activity of arsenic trioxide as an anti-leukemic agent is currently being investigated in many types of leukemia. Although the results look favorable in terms of the response rate of some of the leukemia types that are being investigated, systemic toxicity of arsenic trioxide is a problem (Soignet et al., 1999; Wiernik et al., 1999; Geissler et al., 1999; Rousselot et al., 1999).
The only organic arsenical (OA) still manufactured for human use, melarsoprol, has been evaluated for its antileukemic (WO9924029, EP1002537) properties and showed significant activity. However, this compound is excessively toxic to patients with leukemia at drug concentration and schedule used previously for treatment of trypanosomiasis. Therefore, there is a need to identify arsenic derivatives that can be used for the treatment of hematologic malignancies and cancer in general, that have similar or greater activity and lower toxicity than arsenic trioxide. Organic arsenic derivatives may fulfill this promise as they should be less toxic than the inorganic arsenic trioxide.